1. Field of the Invention
The present invention relates to an electric power transport system comprising a cold dielectric superconducting cable.
2. Description of the Related Art
The term “superconducting cable” denotes a cable for transmitting current in conditions of so-called superconductivity, i.e. in conditions of almost null electric resistance. See, for example, Engelhardt J. S. et al., Application Consideration for HTSC Power Transmission Cable, 5th Annual Conference on Superconductivity and Application, Buffalo, N.Y., Sep. 24-26, 1991. In particular, the term “cold dielectric superconducting cable” indicates a cable showing two concentric phases (conductor and return) in superconducting material, electrically isolated one from the other by a dielectric kept at very low temperature. Such a cable shows an inner channel wherein a cryogenic fluid flows under high pressure.
The above said cable is included in a container with a tubular section, known as cryostat. In the channel externally circumscribed by said cryostat, where the cable is, a cryogenic fluid flows under high pressure (typically, the same fluid flowing into the inner channel of the cable), while the body of said cryostat comprises two coaxial tubes delimitating a gap kept under vacuum wherein, preferably, a thermal insulator is provided.
The term “superconducting material” indicates a material, e.g. special ceramics based on mixed oxide of copper, barium and yttrium (generally known as YBCO), or of bismuth, lead, strontium, calcium and copper (generally known as BSCCO), comprising a superconductive phase with substantially null resistivity at temperature values equal of lower a threshold value, defined as critical temperature (Tc). For example, for the material mentioned above, the Tc ranges from about 60 K (−213° C.) and about circa 170 K (−103° C.).
The operative temperature of a superconducting cable is lower than the critical temperature of the superconducting material contained therein, so as to guarantee a safety margin in case of disfunctioning of the structures for setting and maintaining the proper thermal conditions. These structures mainly include a cryogenic fluid flowing into one or more channels, and a cryostat.
Generally, the cryogenic fluid is helium, nitrogen, hydrogen, argo or mixture thereof, at the liquid or gaseous state, and operates at temperature and pressure specific for the application. Typically, said cryogenic fluid is used under pressure for compensating the pressure fall along the cable length, and for ensuring the right operative temperature. In the case of liquid nitrogen, the temperature under which it is kept is of about 15 atm (about 15,200 mbar).
As already said, the cryostat generally comprises two coaxial tubes, and between them a gap under vacuum is present, preferably at least partially filled with a thermal insulator. A general description on the cryostats used in this field is provided by IEEE TRANSACTIONS ON POWER DELIVERY, vol. 7, No. 4, October 1992, page 1745-1753.
One of the problems associated to the operation of this kind of cable just relates to the maintenance of the vacuum inside said gap of the cryostat The vacuum level suitable for ensuring the cryostat efficiency is rather high, of about 10−6-10−5 mbar. As said above, the cryostat externally delimitates a channel where the cryogenic fluid is made to flow under pressure. Thus, there is a great pressure difference (ΔP) between said channel and said gap. This great pressure gap represents a critical point of high importance in view of the following.